1. Field of the Invention
The present invention relates to a light-emitting diode (LED) lamp and its LED lighting apparatus. More particularly, the present invention relates to the multi-level dimming of an LED lamp and its LED lighting apparatus.
2. Description of the Related Art
Lighting dimmer can save energy, but due to the extra cost in installing various types of electronic dimmers, most incandescent lamps and energy-saving lamps, including compact fluorescent lamps and the emerging LED lighting, are not equipped with any dimming measures. In use, they are either fully turned on, or fully turned off. In the majority of situation, for safety concern, it is highly desirable to be able to dim the light, instead of completely turning it off. Shopping malls, stores, schools, hospitals, small offices, hallways, stairways, factories, and warehouses are examples where partially dimmed lighting is strongly desired during after-work or night hours.
A triac dimmer can retrofit a single-pole single-throw (SPST) wall switch without the need to change the existing wall-switch wiring system. In other words, no re-wiring is required. A triac dimmer works by essentially chopping off some part of the AC voltage. This allows the other part of the AC voltage to pass to the lamp. The brightness of the LED lamp is determined by the amount of power transferred to it, so the more the AC voltage is chopped off, the more it dims.
A Triac is basically a three-terminal, solid-state device that operates directly from AC line. When a short pulse current is injected into the gate terminal, a triac is “turned-on”, that is, it can conduct current until the end of the present half-cycle. Assuming the load is largely resistive, as in the case of a incandescent lamp, the triac will resume its off state (commutate itself off) as the load current drops to zero when the ac voltage crosses the zero level at the end of each half-cycle.
FIG. 1 is a schematic diagram showing a conventional triac dimming circuit 20 for an incandescent lamp 10. Dimmer circuit 20 is inserted between the SPST wall switch 12 and the AC1 power line. Resistor R23 and capacitor C24 determine a delay time after a zero-crossing of the AC line voltage. R23 is a variable resistor. The user may control the aforementioned delay time by adjusting R23. Before triac 25 is turned on, there is essentially no current, except a small current flowing through R23 to charge C24. This small current also flows through lamp 10, which has a resistance of several hundred ohms. After a delay time (determined by R23*C24), the voltage across C24 has reached a sufficiently high value, e.g. 30V to cause diac 26 to break down into a low-resistance state. The break-down of diac 26 provides a short current-pulse into the gate terminal (GT) of triac 25, and turns it into a conduction state.
FIG. 2 is a schematic diagram showing some important signals in the conventional triac dimming circuit 20 in FIG. 1. As shown in FIG. 2, by increasing the delay time, one can reduce the conduction angle, φ, of the triac, thereby control the amount of current or power to lamp 10. This scheme of adjusting the delay time (π−φ) to regulate the power available to the load is also known as phase control.
After the firing of triac 25, the voltage across the terminals MT1 and MT2 of the triac drops essentially to zero. Any remaining voltage on C24 is discharged through R23 and triac 25. Please notice that the maximum conduction angle is less than 180 degrees, since there always requires some delay time to allow C24 to accumulate some charge to trigger diac 26.
Conventional triac dimmers have some drawbacks. (1) It degrades the power factor of the lighting fixture when using a triac dimmer. If the dimmer is set to very dim light condition, the lighting fixture's power factor can be as low as 0.20, or worse.
(2) Triac dimmers generate electro-magnetic interference (EMI) and buzzing sound due to the abrupt turning on the triac in the middle of an AC cycle. Usually some EMI filter means is necessary to suppress the EMI noise from reaching and affecting the operation of other electric or electronic appliances connected to the same wiring system. FIG. 1 shows a typical EMI filter using a capacitor 21 and an inductor 22.
(3) The triac dimmer was originally designed to control a resistive load such as the incandescent lamp. The incandescent lamp emits light through heating its tungsten filament to over 3,000 degrees centigrade. Chopping off some portion of the AC voltage will not appear flickering to human eyes because the filament temperature will not fluctuate even when the AC voltage is pulsating at 60 Hz/120 Hz. However, dimming will make an incandescent lamp operate at lower temperature and emit reddish light.
(4) The triac dimmer is usually NOT compatible with most fluorescent lamps and CFLs (compact fluorescent lamps). It often causes flickering or outright lamp damage. In fact, fluorescent lamps are notoriously difficult to dim.
(5) The triac dimmer is also NOT compatible with most LED lighting fixtures. An LED emits light only when it is conducting a forward current. Once the current is turned off, it stops emitting light immediately. So a chopped AC voltage waveform by using a triac dimmer will result in visible flickers when the conduction angle is less than 90 degrees.
FIG. 3 is a schematic diagram showing a conventional electronic dimmer using an infra-red (IR) remote control system. A conduction modulator 31 is used to vary the power to a lamp 35. An AC/DC power supply 32 provides a DC voltage to an IR receiver 33 and conduction time modulator 31. IR receiver 33 responds to the dimming level command issued by an IR remote controller 36. Please notice that remote controller 36 requires a battery to supply the operating voltage to run its internal circuitry.
Electronic dimmers using infra-red or wireless remote control can provide dimming as well as other sophisticated control functions such as operating time scheduling. It can circumvent the limitations of triac dimming circuits and can support the dimming of fluorescent lamps and LED lamps. But all of these benefits come at a considerably high cost, including (a) extra cost in adding an IR or wireless receiver and a decoder circuit to each lamp. (b) The remote controller unit needs some DC voltage source, such as a battery. (c) The controller is prone to be misplaced, abused, or stolen.
LED lighting is generally operated at a constant current condition. For off-line applications, a buck converter can be used to convert the rectified AC voltage to the constant DC current required by the LED lamp. FIG. 4 is a schematic diagram showing an LED lighting driver 40 based on a buck converter. LED driver 40 includes a switching controller 50, an inductor 47, a free-wheeling diode 48, a power metal-oxide-semiconductor field-effect transistor (MOSFET) 45, and a current sense resistor 46.
MOSFET 45 turns on when a high-frequency clock 53, typically running at over 50 kHz, issues a pulse to set the SR flip-flop 54. Gate driver 55 amplifies the output of SR flip-flop 54 to drive power MOSFET 45. With MOSFET 45 turning on, current flowing through LED lamp 49 and inductor 47 builds up higher. The LED current level is sensed by sensing resistor 46. When the sense voltage across resistor 46 exceeds the reference voltage VREF, comparator 56 resets the output of flip-flop 54. MOSFET 45 turns off accordingly. The inductive LED current flows through diode 48. Inductor current starts to decay until the next clock pulse. By repetitively turning on and off MOSFET 45 at high frequency, the LED current is regulated at a constant level set up by the reference voltage VREF.
Switching controller 50 also includes a dimming circuit, which includes a dimming sawtooth generator 51 and an analog comparator 52. Dimming sawtooth generator 51 typically provides a pulse-width modulation (PWM) dimming frequency (i.e. the dimming sawtooth wave) in the range of 100 Hz to 1 kHz. The dimming voltage VDIM is provided by, for example, an infra-red receiver such as the one shown in FIG. 3.
Changing the level of the dimming voltage VDIM can modulate the conduction time of the LED driver as shown in FIG. 5. From T1 to T2, VDIM is set up at a higher level, for example, 4.5V. The MOSFET is in switching mode all the time. After T2, VDIM level is reduced to 2.0V. When the dimming sawtooth wave rises above the 2.0V VDIM level, at T3, the output of comparator 52 goes low. This will cause AND gate 57 to inhibit clock 53 from setting flip-flop 54. Therefore, the switching operation of MOSFET 45 stops.
At T4, the sawtooth wave drops to zero, and the output of comparator 52 goes high again, enabling clock 53 to set flip-flop 54. Please notice that the duty cycle or the proportion of MOSFET 45 in switching mode during the period from T2 and T4 (i.e. the switching duty cycle of MOSFET 45) is 50%. After T4, VDIM level is further lowered to 1.0V. Likewise, the switching duty cycle of MOSFET 45 during the period from T4 to T6 is 25%. Further reducing VDIM level to 0.5V will cut down the switching duty cycle to 12.5%.
The bias voltage supply circuit 41 includes a bias winding 44 which is coupled to inductor 47. The AC voltage induced by bias winding 44 is rectified and smoothed by a diode 43 and a capacitor 42. Capacity 42 holds sufficient charge to maintain the buck converter in a standby condition when the PWM dimming inhibits the switching of the buck converter periodically. A start up resistor Rst is connected between the voltage VDC and capacitor 42. Resistor Rst helps to kick start LED lighting driver 40 when an AC supply is connected to the LED lamp 49 and the driver circuit initially.
However, conventional dimming control methods using triac device or IR remote control all have some drawbacks. Triac dimmers are relatively low-cost and easy to install, but they are not compatible with LED lighting by their nature. On the other hand, it is clear the bottleneck for other dimming schemes is the extra hardware and cost in transmitting the VDIM information to the LED lighting drivers.